In 1981, we will continue our study of the organization of both the main ribosomal repeat unit of N. crassa (that which codes for the 17S, 5.8S, and 26S rRNA's), and of the 5S rRNA genes. In the case of the main rDNA repeat unit, we will characterize the differences we have found in the non-coding portions of the repeat unit in about 10 different strains we have examined. Dr. Russell has putative clones of these, and we plan to do restriction mapping. Then, if the results are as we anticipate, we plan to sequence the relevant regions of several of them. In addition, we now have on hand the materials and the specific probes needed to clone the first and last repeat unit in the nucleolus organizer region. We plan to proceed with this during the first part of the calendar year. I am particularly interested in the flanking sequences of the 5S rDNA genes, of which there are many different kinds. According to one hypothesis (Selker, Driftmier, Metzenberg, Yanofsky, DeWeerd, and RajBjandary, manuscript in preparation), a degree of homogeneity of the scattered 5S rDNA genes could be maintained by rather frequent transpositions. There are some relatively straightforward ways to test this, and these will be tried. Last but not least, I hope to pursue my study of the phosphorus control genes, which have proved balky at the molecular level. I think we now have built up the skills on somewhat simpler and more tractable things to give a renewed push to this complicated set of genes.